Dispersions of particles of an inorganic oxide such as silica, alumina, zinc oxide, tin oxide, zirconia, and titania are employed in various industrial fields, particularly in the field of optics for modulating refractive index. Among those oxides, zirconia is employed preferably for increasing the refractive index of an optical material because it has a high refractive index.
Such a dispersion of inorganic oxide particles which has been employed conventionally contained water as its dispersion medium, i.e., as an aqueous dispersion, and most of the optical material applications such as optical film production utilizes such an aqueous dispersion usually as being mixed with resin components. However, such an aqueous dispersion is kneaded only with difficulty in particular with a water-insoluble resin component. Accordingly, a dispersion of which dispersion medium is an organic solvent is becoming highly demanded in these days. The inorganic oxide particles including zirconium oxide particles are generally dispersible satisfactorily in aqueous solvents, but are poorly dispersible generally in organic solvents.
Accordingly, it is proposed that an organic solvent is added to an aqueous dispersion of zirconia particles in the presence of a zirconia stabilizer such as acetic acid thereby replacing water as a dispersion medium of the aqueous dispersion with the organic solvent to obtain an organic solvent dispersion of zirconia particles (see Patent Literature 1).
Nevertheless, the organic solvent dispersion of zirconium oxide particles is not sufficiently stable when it contains only a zirconia stabilizer such as acetic acid. Accordingly it is well known that when an organic solvent dispersion of zirconium oxide particles is prepared, the zirconium oxide particles are subjected to surface treatment with a surface treatment agent such as a silane coupling agent thereby making the particles lipophilic, and are then dispersed in an organic solvent.
For example, a method was proposed in which a silane coupling agent is added to an aqueous dispersion of zirconium oxide particles to surface-treat the zirconium oxide particles, followed by solid-liquid separation, and heating and drying to obtain a powder of zirconium oxide, which is then dispersed in an organic solvent thereby obtaining an organic solvent dispersion of zirconium oxide particles (see Patent Literature 2).
Similarly, a method was proposed in which a silane coupling agent is added to a dispersion of zirconium oxide particles in a mixture of solvents of water and methanol, and the solvents are removed without conducting a process of replacing the solvents, and the resultant is dried to obtain a powder of surface-treated zirconium oxide, which is then dispersed, for example, in a ketone solvent thereby obtaining an organic solvent dispersion (see Patent Literature 3).
A method was also proposed in which zirconium oxide particles of which surfaces are coated with a long-chain fatty acid such as neodecanoic acid are prepared, and are then dispersed in an organic solvent, and further coated with a silane coupling agent, subjected to solid-liquid separation thereby obtaining zirconium oxide particles of which surfaces are coated with two different coatings, which are then dispersed in an organic solvent thereby obtaining an organic solvent dispersion (see Patent Literature 4).
However, even when such a powder of surface-treated zirconia particles is obtained and is then dispersed in an organic solvent, the powder of zirconia particles generally undergo substantial agglomeration and is re-dispersed with great difficulty, and the resultant dispersion is poorly stable and its transparency is not necessarily sufficient.
Accordingly, a method was also proposed in which inorganic oxide particles are surface-treated with a hydrolyzate and/or partial condensate of a silane coupling agent in the presence of an acid catalyst in an alcohol solvent having a low ketone solvent content, and thereafter the alcohol solvent is replaced with an organic solvent having a higher ketone solvent content thereby achieving solvent replacement while the surface-treated inorganic oxide particles are dispersed in the organic solvent thereby to obtain an organic solvent dispersion of the surface-treated inorganic oxide particles, in particular, a ketone solvent dispersion of silica particles (see Patent Literature 5).
A further method was also proposed in which an alcoholic solvent dispersion of zirconium oxide particles is preliminarily surface-treated with a silane coupling agent in the presence of an organic acid, and then the alcoholic solvent is replaced with a more lipophilic organic solvent such as a ketone solvent thereby obtaining an organic solvent dispersion of zirconium oxide particles of (see Patent Literature 6).
However, the above-mentioned known methods are not successful in providing an organic solvent dispersion having a sufficient transparency depending on applications. In addition, the organic solvent dispersion obtained suffers poor stability such as gradual increase in viscosity. Such being the case, a higher performance of an optical material is becoming highly demanded in these days, and an organic solvent dispersion of zirconium oxide particles which has a low viscosity and is superior in stability and transparency is highly demanded.